A technique for bonding integrated circuit chips to substrates known as "flip chip" bonding is characterized by an array of solder bumps on a lower surface of the chip and a matching array of solder bumps or bonding pads on an upper surface of the substrate. The solder bumps of the chip are brought into contact with the array on the substrate and sufficient heat and pressure are applied to make a permanent bond of the elements of the two arrays.
For mass production of such devices, it is generally required that the substrate be held by a stationary substrate holder, with the integrated circuit mounted in a vertically moveable vacuum holder which brings the integrated circuit into contact with the substrate. After bonding has been completed, the vacuum of the vacuum holder is released so that the integrated circuit is left bonded to the substrate. This procedure may be robotically controlled and is generally repeated in the production of numerous identical devices.
As circuit densities have increased, the number of solder bumps on each chip has increased and the size of each one has become smaller. Consequently, it has become increasingly important that the vacuum holder be properly registered with the substrate holder. That is, it is important that the integrated circuit be held by the vacuum holder such that its lower surface array of solder bumps lies in a plane which is parallel to the plane of the corresponding array on the substrate.
The copending application of L'Esperance et al., Ser. No. 07/691,658, filed Apr. 26, 1991, hereby incorporated by reference herein, describes a method for using television cameras to align the solder bumps of a chip with those of a substrate. The arrangement permits robotic alignment, but a condition for proper operation of the apparatus is that the horizontal plane of the chip holder be in a plane parallel to that of the substrate holder.
Accordingly, in apparatus for placing an article such as an integrated circuit chip on a substrate with very fine placement accuracy, there is a long-felt need for a technique for assuring that the chip is held in a plane that is parallel to the plane of the substrate.